The basis applied to the distributions despite you not having claimed it, so you must prepare amended Forms 8606 to determine your remaining basis even if you do not submit amended 2018 and 2019 tax returns. Some particular amount of basis was distributed even if incorrectly reported. You do not have the option to distribute basis only when you want to, the tax code does not permit that, which is what you would be doing if you just tried to carry all of your $12,000 of basis forward to 2020.
Since you need to prepare the amended Forms 8606 and the change affects your tax liability,, there doesn't seem to be a good reason not to file the amendments. Your amendments will each include Form 1040-X as well as the corrected Form 8606.
